Separation or detection of ionic species is generally performed based on the electrochemical properties of analytes. Capillary electrophoresis (CE) is an analytical separation technique for analysis of a sample performed in a narrow diameter microchannel or capillary tube, which is filled with an electrically conductive medium (e.g., electrolyte). In electrophoresis, electrically charged analytes move in the electrolyte of the capillary under the influence of an electric field. CE can be used to separate ionic species by their charge, frictional forces, and mass so as to separate species based on their size-to-charge ratio. For example, ionic species in the sample move from one electrode toward another at a rate that is dependent upon certain characteristics, such as molecular charge, size and/or mobility.
CE may be performed using gels or liquids, such as buffers, in the capillary. For example, CE can also be used with fluorescence detection to obtain a high separation resolution, and good signal-to-noise ratios using integrated fluorescence detection optical elements. As compared to electrochemical detection techniques, fluorescence detection techniques are comparatively free from high separation voltage interferences. However, fluorescence detection may require sample to be fluorescent or made fluorescent by virtue of a suitably attached tag. This requires some chemical treatment that leads to the possibility of alteration of physical and chemical characteristics of the target molecule.
An alternative technique for detection without modifying the sample is optical absorbance. For absorbance detection, ultraviolet (UV)-visible range light is used because biomolecules have significant absorption at such wavelengths. Absorption based detection uses a light path across a microchannel cross section, and may be limited by a path length in terms of sensitivity and detection resolution and scattering of light as the light passes through the microchannel. Micro-lenses may be used to focus the light into the microchannel and collecting the light at a detector.
Evanescent light wave absorbance based techniques may also be used in which the sample interacts with the evanescent field of light traveling along a waveguide (optical fiber). The interaction induces changes in absorbance characteristics between a source and a detector placed at opposite ends of the waveguide.
Optical detection methods may have a broad application in analytical biology and chemistry. Optical coupling of microfluidic elements in microstructure analytical devices can be performed for use in optical detection methods. Microfluidic channels may need to be hermetically sealed, and a location and position of optical fibers can result in complex designs that are difficult to manufacture.